Apparatus functioning according to such a principle is illustrated in U.S. Pat. No. 4,107,606. This publication only illustrates the said principle very schematically, and is more specifically directed towards a construction for electrical distance measurement between two measuring heads. How these are to be designed and implemented is not described in detail particularly as to the maintenance of distance provided pneumatically.
Another apparatus of the same kind is disclosed in U.S. Pat. No. 3,528,002, although what is described in detail really consists of a fixed measuring head, from which the web is caused to maintain a constant distance while only the opposing head is movable. The distance maintenance is described as taking place by compressed air being caused to emanate through a porous plug, for generating an air cushion and a lifting force counteracted by a constant counterforce, which is illustrated as being generated by a relatively heavy mass. It is obvious that this is a great disadvantage if a rapidly-acting thickness meter is desired, since large mass results in large inertia.
Another prior art method for measuring thickness utilizes a single movable transducer, the position of the web being maintained fixed against an underlying plate, e.g., a suction plate, while the movable transducer is kept by means of a pneumatic distance maintenance at a constant distance from the other side of the web. Such a device is known from U.S. Pat. No. 3,818,327, the distance between the suction plate and transducer being measured electromagnetically. A similar device is known from U.S. Pat. No. 3,617,872. With regard to the movable transducer, it is disclosed in both cases that an air cushion generated by compressed air is used together with a relatively heavy structure, the gravitational force of which counteracts the air pressure.
A substantially lighter structure for a transducer having pneumatic distance maintenance is known from applicant's own Swedish Published Specification No. 7900795-1. In this structure there is a light, movable and hollow measuring means, which is carried in air bearings, is axially movable in a holder, has at one end an air-blowing nozzle and at the other end a disc arranged with annular clearance in an outwardly open cylindrical chamber, the portion of the cylindrical chamber partially closed off by the disc being supplied with compressed air such that a counterforce acts on the measuring means for urging it in a direction away from the measured surface. This structure functions relatively well, but it would be desirable to make it faster in assuming its state of equilibrium, and to reduce the distance between nozzle and measuring surface.